Duty Tax

[Wompet]

If, on a seemingly regular basis, the local Post Masters (agents of the federal government) do not see fit to collect duty fees...

Small note of correction: The U.S. Postal Service does not collect duty. U.S. Customs collects duty after determining the correct tariff and rate for the item in question.
All shipments coming into the U.S. go through a screening process. Shipments in the mail make entry at one of the international mail facilities, where they are reviewed by U.S. Customs officers for potential duty, narcotics, currency, disease agents, midget smuggling, and other contraband. If it's determined that duty should be collected, it's Customs that holds the package and sends you the notice. You may have to pay, and you may not - general experience here is that you won't.
With commercial carriers, all of the shipments are still reviewed, but they may pre-emptively charge you duty - just in case. So Customs may decide that you don't have to pay, but the carrier says you do. And the carriers have been known to use the incorrect tariffs (intentionally or not) to collect a higher rate of duty.

Now, to go back to the original question:

Anyone give me an idea of what I am facing for a duty tax for a kilt coming into the US?

I've ordered two kilts from Lochcarron, two more from eBay UK, and even yardage from Marton Mills, and I have never been charged duty. All of these were shipped Royal Mail.

The closest tariff I can find is 6104.19.5000, "Women's ... skirts ... of wool or fine animal hair" (there isn't a tariff for kilts, so this is the next closest garment) - duty is 13.6%, if collected.

But:

... also request that they list it on the customs label as "Ethnic Clothing" which is supposed to go through without (much if any) duty.

These items are supposed to be duty free (I'm not finding the exact tariff at the moment ).